Process for improving electrical and thermal contacts



United States Patent 3,439,080 PROCESS FOR IMPROVING ELECTRICAL AND THERMAL CONTACTS Leonard M. Vaught and Lee Roy Cervenka, Lake Jackson, Tex., assignors to The Dow Chemical Company, Midland, Mich, a corporation of Delaware No Drawing. Filed Apr. 4, 1966, Ser. No. 539,634 Int. Cl. C04b 35/54 US. Cl. 264-105 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method for improving the electrical and thermal efficiency of electrical and thermal contact surfaces, such as between clamps and electrodes, which comprises inserting malleable graphite between such surfaces in a quantity which is at least sufficient to occupy any areas of such surface where there is otherwise no contact.

This invention relates to a process for improving the efliciencies of electrical or thermal contacts and more particularly relates to a process for increasing the electrical and thermal efficiency of metal clamps such as are used as connectors to graphite anodes.

The problem of obtaining and maintaining good electrical contact between an electrical clamp and an electrode has long been recognized and numerous solutions have been proposed therefor. Such solutions include the use of carefully mated and machined electrode and clamp urfaces and the use of increased clamping pressure. Also this approach has proven time consuming and unduly expensive, and has not provided a highly eflicient clamping means.

It is an object of this invention to provide a process whereby the efiiciency of electrical and thermal contacts may be improved. A further object is to provide a process whereby electrical clamps may be contacted with electrodes to provide improved electrical and thermal efiiciencies. A further object is to provide a method whereby such increased thermal and electrical efliciency may b quickly, easily, and economically achieved. These and other objects and advantages will become obvious from a reading of the following detailed description.

It has now been discovered that the electrical and thermal conductivity at the interface between electrical or thermal contact faces may be greatly enhanced by providing between such contact surfaces a quantity of malleable graphite either in the particulate vermicular form or in partially compressed form. Such graphite is placed between the contact surfaces prior to the application of clamping pressure. Under usual clamping pressure, the malleable graphite flows to fill any spaces between the contact faces to thereby provide a highly conductive contact where substantially less contact would otherwise have been present.

The vermicular graphite used herein is a compressible form of expanded graphite prepared by introducing an intercalating agent between the laminae of natural or synthetic graphite and expanding such treated graphite by heating. For example, a heat-expandable graphite may be prepared by contacting particles of natural flake graphite with fuming nitric acid, fuming sulfuric acid, mixture of concentrated nitric and sulfuric acids, perhalo acids, or the like, for a period of a minute or so. The treated graphite particles may then be washed free of excess intercalating agent and dried if desired. The heat-expandable graphite thus prepared may be expanded from 20 to 600 times, usually between 200 and 600 times its original volume by heating, e.g. to 500 C. or more. Such expanded graphite is usually in light weight particulate, vermicular, wormlike form, usually has a bulk density of 0.005 to 0.01

gm./ cc. and is easily malleable and compressible into any desired shape up to a density approaching the theoretical of about 2.26 gm./cc. This light weight vermicular form of graphite may be compressed into compacts having a density of up to about 0.07 gm./ cc. and while still retaining its ability to deform and flow under relatively low pressures, e.g. pressures of 15 p.s.i. to psi.

The term malleable graphite when used herein, therefore, refers to vermicular graphite in either particulate form or in the form of a compact having a density up to about 1.9 gm./ cc. and which is deformable under pressure.

The amount of malleable graphite employed between two contact surfaces such as between a clamp and an electrode depends largely upon the roughness and irregularity of the surfaces to be joined. For the optimum efliciency, suflicient malleable graphite is employed to fill all gaps and discontinuities between the clamp and the electrode upon tightening the clamp, and leave little if any malleable graphite between the clamp and the electrode at points of natural contact. For example, when employed with serrated clamps, as frequently used to conduct current to electrographite anodes for the electrolytic production of magnesium metal, suflicient malleable graphite is used to fill the notched portions of the clamp face to thereby give a more complete contact between the clamp face and the anode surface. Preferably, however, little or no malleable graphite remains between the clamp teeth and the electro graphite to reduce the normal contact therebetween. Malleable graphite employed in this manner greatly increases clasping efliciency, reduces voltage drop across the clamp interface and improves heat dissipation from the electrode.

Additional advantages are achieved by employing malleable graphite between an electrode and the clamp therefore. If, as in many electrolytic processes where electrographite electrodes are employed, corrosive gases are given off, such gases pass through the electrographite and attack the face of the clamp. This causes loss in conductiv ity, periodic shutdown to clean the clamp and, eventually clamp destruction. This problem is greatly reduced or eliminated by the use of malleable graphite which is highly impervious to gases and does not itself corrode.

Another problem frequently encountered with electro graphite is the necessity of completely removing the clamp before the electrode can be positioned by moving it up or down within the cell or bath. Malleable graphite placed between the clamp and the electrographite provides sufiicient lubricity to enable the clamp to be merely loosened and the electrode moved up or down without damage.

Malleable graphite, if in cohered or compacted form, may be wrapped around the electrode and the clamp placed around the graphite layer. If the malleable graphite is in particulate, vermicular form, it is applied between the clamp and the electrode prior to tightening of the clamp.

The electrical junction resistance of battery cathodes is likewise improved by employing cathodes containing at lea st an outer surface of compressed vermicular graphite. Such graphite provides a more eflicient contact surface between the cathode and the depolarizing mixture and thereby lowers the junction resistance at the cathode face. Such efliciency is even further increased 'by employing a depolarizing mixture which contains particulate vermicular graphite.

The following examples are provided to more fully illustrate the invention but are not to be construed as being limiting thereto.

Example 1 1" x A" x 10 /2" piece of commercial electrograph- 1te impregnated with phenol-formaldehyde resin was employed as the electrode in the following tests. At right angles across the 1 inch face of the electrograph'ite was clamped a flat steel plate 1 inch wide. The 1 inch square portion of the steel plate which contacted the electrographite was sanded to provide good contact. Clamping 4 Example 4 In order to demonstrate the usefulness of this invention under actual commercial conditions, one of the circular steel clamps used to supply current to electrofofce between the steel Plate and ths electrode was 5 graphite anodes in an electrolytic cell for the production tamed and measured y a force 3 13 constant of magnesium was modified according to this invention. current flow of 1.35 amps was maintained between the Such clamp was sandblasted and a layer of malleable electrode and the steel clamp. Voltage drop between such graphite foil 0077 in thick having a density of 1' 4 i i i anixilectrode was measured between Contact points 10 cc. was placed between the clamp and the anode with the 111C ap In separate, but otherwise similar experiments, various conductwe plane of the 011 bemg parallel to the clamp thicknesses and forms of malleable graphite were inserted face between such electrode and the one square inch of metal All clflmps employed? Serrated faces Wlth surface Contact area of the clamp and Similar Voltage drop grooves similar in profile to National course threads on a measurements were made. Thus the results of such uses of 3/1 dlameter bolt- A11 clamps Wsre sandblasted l malleable graphite were compared with the use of no before use and clamped to the anode With a force of malleable graphite. The results obtained are shown in the about 700 p.s.i. Each clamp conduc ed a ut amps following Table I. (DO) per square inch of clamp surface. The anodes TABLE I Millivolt drop between clamp face and electrode at following clamp @est Malleable graphite between clamp and electrode pressures, p.s.1.

Control N 72. 5 45.6 34.9 28.4 23.3 20.3 17.3 15.8 14.5 13.4 1 0.017 in. thickness of uniaxially compressed foil with high 62.4 53.2 45.2 38.7 32.4 26.6 22.8 20.3 18.6 16.8

conductivity axis parallel to clamp face (1.82 gm./cc.). 2 0.032 in. thickness of uniaxially compressed foil with high 50.7 33.5 25.7 22.0 18.1 16.4 15.2 12.9 12.1

comliucgivity axis perpendicular to clamp face (1.8 gm. cc. 3 vermicular graphite having density of 0.005 gm./ec. Final 22.8 15.4 12.4 9. 5 8. 5 7. 6 6. 9 6.4 6. 1 5. 6

average thickness after 100 p.s.i. clamp pressure was 0.007 in.

Example 2 were partially immersed in a 700 C. molten salt bath containing MgCl As necessary the anodes were lowered into the molten salt bath and the clamps were moved up the anode stem.

For purposes of comparison, Clamp A with malleable graphite foil and Clamp B without such foil (control) were operated in parallel and the voltage drop between TABLE II Millivolt drop between clamp face and electrode at following clamp ilest Malleable graphite between clamp and electrode pressures, par.

Control-.- None 74.1 60.7 45.2 35.2 20.2 24.8 22. 5 20.4 18.7 17.7 1 0.112 in. thickness of uniaxially compressed foil having a 70.0 36. 5 22.4 17. 5 15.4 14.3 12. 9 11. 7 10. 7 9.9

density of 0.06 gm./cc. and high conductivity axis parallel to clamp face.

Example 3 each clamp and anode was measured. Results are shown In order to demonstrate the increased thermal conm the followmg Table ductivity between two conductive materials when malle- 50 TABLE Iv able graphite is employed between such materials, a 40 p.s.i. clamping force was employed between two sanded, Days operation Vdtage drop mmmvolts smooth, electrographite blocks. The upper block is 5 11 11 1 ClanfipilB designated as Block A and the lower block as Block B. In one instance no malleable graphite was employed .32% between the blocks and in the other instance a layer of vermicular graphite having an initial bulk density of 0.005 8. 8g; gm./cc. was employed which after compression between 355 the blocks formed a film having a thickness of 0.005 inch. An electrical heater was applied to the upper surface of the upper graphite block and the central temperatures of each block was recorded as shown in Table 111.

1 The higher initial resistance of clamp A was due to the use of a thicker graphite foil to demonstrate the effect of a corrosive atmosphere upon camps with and without such graphite foil.

At the end of 45 days, the clamp having no foil showed extensive evidence of corrosion on the clamp face while the clamp containing graphite foil showed no significant quantity of corrosion.

Various modifications can be made in the present invention without departing from the spirit or scope thereof for it is understood that we limit ourselves only as defined in the appended claims.

We claim:

1. A process for improving the electrical and thermal efiiciency of electrical and thermal contact surfaces which comprises applying vermicular graphite in particulate form or in compacted form having a density up to about 1.9 gm./cc. between said surfaces in a quantity at least suflicient to occupy any areas of said surfaces where there is otherwise no contact.

2. The process of claim 1 wherein the graphite has a bulk density of up to about 0.07 gm./ cc.

3. The process of claim 1 wherein the graphite is in cohered anisotropic form.

4. The process of claim 1 and including the step of applying sufiicient pressure to said surfaces to cause deformation and flow of the graphite between such surfaces.

5. A process for providing improved electrical and thermal contact between metal clamps and graphite anodes which comprises inserting between such clamp and such anodes a quantity of vermicular graphite in particulate form or in compacted form having a density up to about 1.9 gm./cc. sufiicient to form a graphite layer of between about 0.0005 inch and 0.5 inch thick when subjected to clamping pressure and exerting a clamping pressure of at least 15 p.s.i. upon such graphite.

6. The process of claim 5 wherein the graphite has a density of from about 0.005 gm./cc. to about 1.9 -gm./cc. prior to being subjected to clamping pressure.

7. The process of claim 5 wherein the graphite is in vermicular form prior to exerting said clamping pressure thereon.

References Cited UNITED STATES PATENTS 1,312,255 8/1919 King 264-105 2,907,705 10/1959 Blainey 264-27 3,044,151 7/1962 Coler 264-104 3,250,832 5/1966 Metz 264-27 FOREIGN PATENTS 991,581 5/1965 Great Britain.

DONALD J. ARNOLD, Primary Examiner.

US. Cl. X.R. 

